The present invention relates to a rolling-body screw drive having a threaded spindle and a threaded nut enclosing the threaded spindle, a helically running threaded channel provided between an outer circumferential surface of the threaded spindle and an inner circumferential surface of the threaded nut, the threaded channel forming, together with a return channel which connects the two end regions of the threaded channel, an endless circulatory channel in which an endless series of rolling bodies is accommodated, and in which each of the two end regions of the threaded channel is assigned a deflecting element, which is retained on the threaded nut and has a deflecting channel, for transferring the rolling bodies between the threaded channel and the return channel and between the return channel and the threaded channel.
Such a rolling-body screw drive is known, for example, from DE 29 14 756 C2. It can easily be recognized that the task of producing those deflecting elements of the rolling-body screw drive which have a closed deflecting channel involves a relatively high cost outlay since the injection mold, for forming the deflecting channel, has to have at least two core elements inserted into the mold cavity from different sides. It is necessary for these core elements, once the deflecting element has cured, to be removed from the deflecting element and, in order to produce the next deflecting element, to be reintroduced into the mold and repositioned there. Furthermore, the deflecting elements of the known rolling-body screw drive, in particular in the case of heavy-load embodiments with large rolling-body diameters, were occasionally found to be insufficiently robust. This resulted in practice, in particular in the embodiments with an open deflecting channel, in undesired deformations of the deflecting elements on account of vastly differing wall thicknesses.
Deflecting elements as have been described above are also known from DE 24 37 4,7 C2. Reference is made in that regard to a brochure from A. Mannesmann, Remscheid entitled xe2x80x9cAM-Werknorm 26xe2x80x9d (AM Works Standard 26), which discloses the production of a deflecting element with a deflecting channel that is not fully closed in the circumferential direction. The deflecting channel is formed by means of an end mill which has cutting edges provided in spherical form on the tip.
In contrast to the foregoing, the object of the present invention is to provide a rolling-body screw drive of the type mentioned in the introduction with deflecting elements which are easier to produce.
This object is achieved according to the invention by a rolling-body screw drive of the aforementioned type in which at least one of the deflecting elements is made up of at least two deflecting-element parts which together form the deflecting channel. In the preferred production of the deflecting-element parts as plastic parts using injection molding, there is thus no need for any cores to be inserted into the mold in order to form the deflecting channel. It is thus possible for a completely cured deflecting-element part to be demolded simply by virtue of the mold halves being opened and for the mold to be prepared for the production of the next deflecting-element part simply by virtue of being reclosed. Cutting-type follow-up machining of the deflecting-element parts is not necessary. In particular, when a boundary surface of each of the deflecting-element parts, which boundary surface helps to bound the deflecting channel, extends from an inlet end of the deflecting channel to an outlet end of the deflecting channel, a deflecting element with a deflecting channel which is closed over its entire circumference can easily be produced.
The deflecting element may have, for example, a main deflecting-element part and at least one secondary, deflecting-element part. The main deflecting-element part, in addition to bounding the deflecting channel, may also perform other functions, for example, serving for fastening the deflecting element on the threaded nut. The deflecting-element parts may be clamped to one another, for example, or connected to one another by means of at least one pin which is arranged on one of the deflecting-element parts and engages in a corresponding recess in the respectively other deflecting-element part. If a plurality of such pin/recess pairs are provided, not all of the pins have to be arranged on one and the same deflecting-element part. At least one pin/recess pair may be designed such that the pin is introduced into the recess under pressure, with the result that a press fit is produced. Within the context of the present application, this type of connection may also be regarded as xe2x80x9cclampingxe2x80x9d of the deflecting-element parts. In principle, however, it is also conceivable for the deflecting-element parts to be adhesively bonded to one another.
A particularly reliable manner of fitting the deflecting element on the threaded nut to secure these two parts together by a screw-threaded fastener. It is possible here for a through-passage hole to be provided in the deflecting element for the through-passage of a screw-bolt fastener. Preferably, the through-passage hole is located only in the main deflecting-element part. In that case, as a result of the fastening, the at least one secondary deflecting-element part may be retained on the threaded nut by the main deflecting-element part.
In a particular aspect of the deflecting elements of the rolling-body screw drive according to the invention, the penultimate thread turn of the threaded surface bounding the threaded channel on the part of the threaded nut is formed, at least in part, by the outer surface of the deflecting element. In this way, the deflecting element can take up a relatively large amount of space in the threaded nut without adversely affecting the functioning of the latter. This means that the deflecting element may be of robust design, even when rolling bodies of large diameter are used.
If the course of that running-path flank of the penultimate thread turn which xe2x80x9ccuts into xe2x80x9d the deflecting element is selected such that said running-path flank is specifically relieved of loading as the rolling bodies move past, it is possible for wear to which the deflecting element is subjected by said rolling-body movement to be minimized, if not completely prevented. Such specific influencing of the course of running-path flanks is also referred to in specialist circles as xe2x80x9cshiftingxe2x80x9d.
In a further aspect of the invention, the deflecting element may also have a cutout or recess into which there is inserted a retaining element which is formed from a harder or softer material than the deflecting element. The retaining element is dimensioned in the direction of the depth of the cutout such that the retaining element accommodated in the cutout projects from the cutout to a greater extent than corresponds to the play or clearance provided between the deflecting element and the recess in the threaded nut which accommodates the deflecting element. It is possible, for example, for the retaining element to project from the cutout by between approximately 0.05 mm and approximately 0.4 mm, and preferably approximately 0.2 mm. In a particularly simple embodiment, the retaining element may be a ball. This ball may be produced from a material which may be either softer or harder than the material of the deflecting element. The ball may thus be produced, for example, from NBR (Nitrile-Butadiene Rubber) or from hardened steel. Use of a softer material, for example NBR, has the advantage that, when the deflecting element is forced into its accommodating recess, rather than the deflecting element being deformed, which could adversely affect the deflecting channel, it is the retaining ball that is deformed.
If the cutout is provided in a sub-surface of the outer surface of the deflecting element which is arranged essentially opposite that sub-surface of the outer surface of the deflecting element into which that end section of the deflecting channel which leads to the threaded channel opens out, it is possible for the deflecting element, as a result of the prestressing by the retaining element, to be forced against the threaded nut such that an essentially flush transition from the threaded channel to the deflecting channel is produced.
In order for it to be possible to facilitate the transfer of the rolling bodies from the threaded channel into the deflecting channel and vice versa, the deflecting element may also have a rolling-body lifting extension, which is intended for engaging in a thread turn of the threaded spindle. In this case, the outer contour of the lifting extension may be designed to correspond essentially to the contour of the threaded surface of the threaded spindle, and preferably such that, in the state in which the rolling-body screw drive is assembled and ready for operation, the outer contour of the lifting extension is arranged in an essentially equidistant manner to the contour of the base surface of the thread turn of the threaded spindle.
In a further aspect of the invention, it is also proposed that the return channel be formed by the interior of a return tube which is designed in one or more parts, has a tube wall preferably with no openings and is inserted into a return cutout or bore of the threaded nut, but preferably does not come into contact with the boundary walls of the return cutout. The use of such a return tube has the advantage that the production of the threaded nut is simplified since all that it requires is to have a return bore dimensioned with relatively large tolerances. A further advantage of using such a return tube is the associated reduction in the development of noise. In conjunction with a device which prestresses the deflecting element as aforementioned, i.e., by urging that end section of the deflecting channel which leads to the threaded channel in the direction of the threaded channel, the provision of a return tube has the additional advantage that any possible displacement of the deflecting element as a result of such prestressing cannot have an adverse effect, in the region of the other end section of the deflecting channel, on the transition between the return channel and the deflecting channel.
EP 0 472 167 31 and U.S. Pat. No. 5,800,064 disclose linear bearings in the case of which use is made of a return-tube arrangement and/or a slit return tube. Until now, however, the use of such return tubes has not been known in rolling-body screw drives.
In accordance with the invention, the return tube may be connected to the deflecting elements, for example, in that at least one of the longitudinal ends of the return tube engages in an annular recess formed on the deflecting element. Preferably, the return tube is clamped to the deflecting element.
It should also be added that the rolling bodies are preferably balls. In principle, however, other types of rolling bodies, for example rollers, should not be ruled out either.